It has long been known in the art that chlorine dioxide has great utility as an oxidizing treatment having a variety of applications. Chlorine dioxide has found use in the paper industry as a bleaching agent for paper pulp, in the water and waste treatment industry as a biocide particularly effective against bacteria and algae, in the fat rendering and tallow industry as both a biocide and bleaching agent and generally as an oxidant useful in destroying certain organic materials, such as phenols.
Numerous methods of generating chlorine dioxide and equipment related thereto have become well known throughout the industry, as set forth in Ullmann's Encyclopedia of Industrial Chemistry, Volume A 6, pages 496-500. One method of particular importance involves the reaction of a mineral acid with an alkaline metal hypochlorite salt and an alkaline metal chlorite salt. This method will be referred to herein as the mineral acid - hypochlorite salt - chlorite salt route.
The mineral acid - hypochlorite salt - chlorite salt route follows the general formula: EQU 2H.sup.+ M+2AClO.sub.2 +AOCl.fwdarw.2ClO.sub.2 +3A-salt+H.sub.2 O
wherein H.sup.+ M represents one or more mineral acids, for example, acids selected from the group consisting of HCl, H.sub.2 SO.sub.4, HNO.sub.3, and H.sub.3 PO.sub.4 ; A represents one or more alkaline earth metals, for example, those metals found in Group 1A of the Periodic Table, i.e., metals selected from the group consisting of lithium, sodium, potassium, rubidium, cesium and francium; and A-salt represents the alkaline metal product salt or salts formed in the reaction.
More specifically, the hypochlorite salts are selected from the group consisting of NaOCl, LiOCl, KOCl, RbOCl, CsOCl and FrOCl. By far, NaOCl is the most preferred alkaline metal hypochlorite salt reactant. The chlorite salts are selected from the group consisting of NaClO.sub.2, LiClO.sub.2, KClO.sub.2, RbClO.sub.2, CsClO.sub.2 and FrClO.sub.2. By far, NaClO.sub.2 is the most preferred alkaline metal chlorite salt reactant.
A-salts are formed by the combination of the alkaline metals dissociated from the hypochlorite and chlorite reactant salts and (1) the anion contributed by the mineral acid; and (2) an anion (a chloride ion) contributed by the hypochlorite and/or chlorite salts. It is believed that the hypochlorite is the predominant chloride ion contributor of the two reactant salts.
It will be recognized by those of average skill in the art that reactant salts having mixed alkaline metals will produce a mixed alkaline metal A-salt product. Furthermore, use of H.sub.2 SO.sub.4, HNO.sub.3, and H.sub.3 PO.sub.4 as the mineral acids will result in a mixture of A-salt products due to the mineral acids contributing anions other than Cl.sup.-.
The most preferred reactants for use in the reaction route discussed hereinabove are hydrochloric acid, sodium hypochlorite and sodium chlorite. Thus the general formula for the reaction route wherein the most preferred reactants are employed is: EQU 2HCl+2NaClO.sub.2 +NaOCl.fwdarw.2ClO.sub.2 +3NaCl+H.sub.2 O.
Heretofore, the mineral acid - hypochlorite salt - chlorite salt route for generating chlorine dioxide required the addition of large quantities of water to keep the alkaline salt by-products of the generation process in solution and also to cool the reaction.
U.S. Pat. No. 4,013,761 describes in detail a mineral acid - hypochlorite salt - chlorite salt process for generating chlorine dioxide and an apparatus therefor. In particular, the '761 patent describes a process wherein the three reactants are fed into a generating chamber to react and form chlorine dioxide and reaction by-products. The '761 patent teaches that relatively large quantities of water in the reactants are an important aspect of the disclosed method. In fact, the '761 patent teaches that water comprise from 95% to about 99.9% by weight or more of the total materials in the generator.
U.S. Pat. No. 4,143,115 describes another mineral acid - sodium hypochlorite - sodium chlorite process and apparatus for generating chlorine dioxide. Again, by weight, water comprises from 95% to 99.9% or more of the composition of materials in the generation vessel.
U.S. Pat. No. 4,247,531 describes an apparatus in which aqueous streams of the three reactants are drawn into a common manifold area by means of a partial vacuum or low pressure induced by a water stream driven venturi eductor. The venturi eductor is powered by a stream of water which also acts as a diluent and carrier fluid for the reaction products of the chlorine dioxide generation process. The amount of water added to the chlorine dioxide product using the process and apparatus of the '531 patent is said to be less than the 99% water dilution found with other generation systems. The '531 patent teaches that materials other than water which are not reactable with and are compatible with ClO.sub.2, such as inert gasses like nitrogen, can be used to both power the venturi eductor, and act as a carrier stream for the chlorine dioxide. However, water, because of its ready availability and low cost would appear to be the most likely carrier material.
In the field, it was found that the manifold area of the apparatus described in the '531 patent would rapidly become clogged with the alkaline salt by-products of chlorine dioxide generation, such as sodium chloride. To alleviate this problem when using the chlorine gas - metal chlorite salt route, a modified version of the '531 apparatus was developed. U.S. Pat. No. 4,590,057 describes a method and apparatus comprising a venturi eductor in communication with a common manifold into which are drawn by a reduced pressure caused by the venturi eductor the reactants for the generation of chlorine dioxide and a stream of bleed water to dilute and carry away the alkaline salt by-products of chlorine dioxide generation.
Yet another method and apparatus of producing chlorine dioxide by the mineral acid - sodium hypochlorite - sodium chlorite route has been known in the art since at least 1979 and is erroneously referred to as the "dry gas method" of chlorine dioxide generation. The "dry gas method" comprises reacting mineral acid, sodium hypochlorite and sodium chlorite in a stream of air and water, wherein the water is fed at a rate of from one quarter to one half gallon per minute. In treating a typical vat of rendered fat or tallow, the chlorine dioxide generator would be required to run for four to five hours, thus introducing from 60 to 150 gallons of water over and above that which is the natural reaction by-product of the chlorine dioxide generation process. It is clearly not a dry process, in spite of its name.
The so-called "dry gas method" was found to be inadequate in treating large quantities of rendered fat and tallow on account of the water added by the treatment process. Specifically, when the chlorine dioxide produced by the "dry gas method" was introduced to the rendered materials prior to centrifugal water separation treatment, the added water would cause the centrifuge to overload, allowing large amounts of water to be carried over into the rendered products. When the chlorine dioxide treatment was conducted in the storage vessel for the rendered material the natural gravity separation of the water was found to take an extremely long time. It is estimated that by standing it may take a week or longer to acheive good water separation with the quantities of rendered materials being treated. Those familiar with the fat and tallow industry will recognize that the presence of water in the rendered product reduces its quality and market value.
Adding water to the reaction stream results in an aqueous chlorine dioxide product stream, which may be acceptable for certain bleaching and oxidizing applications but may also be highly undesirable for others.
The present invention developed as a result of the requirement of a mid-western United States animal by-products processing plant to bleach large batch quantities of rendered fat and tallow by treatment with chlorine dioxide. It was found that the then known methods of generating chlorine dioxide were not entirely desirable to the fat rendering industry because of the large amount of water introduced to the rendered materials as a part of the effluent stream carrying the chlorine dioxide. Any water added to the rendered materials would have to be removed via a separation processes, such as centrifuging, before the rendered materials would be suitable for sale or shipment to customers. The added step of separating the water from the rendered materials would increase the cost of the fat and tallow products and would slow the overall production time. Therefore it was necessary to develop a process for generating chlorine dioxide which would add a minimum amount of water to the materials treated therewith.
To correct the problems associated with treating rendered materials with chlorine dioxide obtained by the "dry-gas method", the present invention was developed. In the present invention, the mineral acid, alkaline hypochlorite salt and chlorite salt reactants are combined in the presence of a moving stream of air, a carrier and reaction medium which is reactable with chlorine dioxide to the extreme degree of being explosive. As the reaction takes place, the reaction products are moved down stream under the influence of the air pressure.